1. Field of the Invention
The invention relates to a process for depositing palladium layers on metal surfaces, a bath for carrying out the process, and the use of the process.
2. Background of the Invention
Base metals can be protected against the attack of aggressive gases or liquids by means of corrosion resistant metal coatings, the type of which is determined essentially by the intended use of the article. For example in a welding wire, iron/steel are protected against rusting by thin copper layers, deposited thereon. In the electronics industry, gold is commonly used for coating surfaces to be bonded or soldered or surfaces for electrical contact. Silver is generally not used for corrosion protection due to its tendency to migrate.
Nickel coatings may also be used for corrosion protection of, for example, copper and copper alloys. First the surfaces are superficially activated. Then the article having the surfaces to be coated is dipped into an acidic palladium solution, so that extremely fine palladium particles are formed, and on which the deposition of nickel starts. The palladium coating is not sealed but is very finely distributed. The palladium coated surfaces have a gray appearance. It is the subsequent nickel coating that seals the surface completely. However, the nickel coating layers are not resistant to oxidation. Thus, the nickel coated surfaces cannot be soldered or bonded after storage and are thus not suitable for providing corrosion protection to circuit boards.
For applications in which the corrosion-protection layer also serves as the final solderable and bondable layer, noble metals are primarily used. Palladium is generally used due to its relatively low cost when compared to other noble metals. Various baths for the chemical deposition of palladium layers are known in the art (e.g., U.S. Pat. No. 4,424,241, U.S. Pat. No. 3,418,143, U.S. Pat. No. 3,754,939, DE-OS 42 01 129, GB-PS 1,164,776, DE-OS 30 00 526, U.S. Pat. No. 4,341,846, U.S. Pat. No. 4,255,194, DE-OS 28 41 584, and EP-0 423 005 A1).
U.S. Pat. No. 4,424,241 discloses a process for the chemical deposition of palladium. The process operates at a pH-value lower than 2 and uses formic acid, among other agents, as a reducing agent. Along with carboxylic acid, amines are disclosed as complexing agents; however, no information is given about the particular type of amines used. Comparative tests show that the palladium layers deposited from these baths are black and do not adhere satisfactorily to the substrate. Furthermore, the baths decompose very rapidly. It is also stated that the danger of spontaneous decomposition of the bath exists when the concentration of the reducing agent is set too high.
U.S. Pat. No. 3,285,754 discloses a cementation bath for the deposition of palladium on copper and copper alloys and other substrates. The bath contains nitrito-palladium complexes and operates in the pH range of between 2 and 5. The palladium complex also contains complex-bound acid anions such as, for example, sulfate, acetate and chloride. However, the layers produced with the bath are extraordinarily thin and contain pores.
German publication DE-OS 42 01 129 discloses a process for producing a wiring board by means of currentless palladium-plating on the copper portions of the board. The process uses known palladium coating solutions as the palladium baths and which contain, for example, hypophosphorous acid, phosphorous acid or hydrated boron compounds but not formic acid as the reducing agent. During the coating process the coating solutions are enriched with reaction products, such as for example, phosphite, phosphate or borate, from the oxidation of the above-described reducing agents. These reaction products cause the deposition conditions to deteriorate due to, for example, increased stability of the bath against self-decomposition.